Clarify semantics of dots in column names

[pivovarit]

Apparently, there's some tribal knowledge that dots in column names should not be used, because they are reserved for nested semantics.

However, users discover it the hard way because it's not reflected in the documentation: example

Tribal knowledge:

• Renaming column with dot . in column name doesn't work. ClickHouse#55452 (comment)
• Non-nested column with dot in name ClickHouse#8193 (comment)
• https://stackoverflow.com/a/65497303/2229438

[nikitamikhaylov]

ClickHouse/ClickHouse#68727